Summary of work: Age-related changes in bone mass have been demonstrated in both men and women. Age and sex related differences hormones, nutritional and physiological variables involved in bone turnover are important in elucidating changes in bone physiology in normal aging and disease. Specifically, the relationship age and sex to changes in body composition, both as a marker of obesity and as a measure of the weight bearing load on bone are important factors in bone status and rates of change. The most dramatic rates of change in bone mass occur at the menopause, and changes of body composition, bone mass, and bone markers and hormones are being investigated in a cohort of perimenopausal women recruited to the Baltimore Longitudinal Study of Aging, who will be followed at 3 month intervals as they traverse the menopause. These studies will allow for the interpretation of the changes that occur prior to and immediately around the time of the cessation of menses, changes that can only be appreciated in a prospective longitudinal study. Preliminary results compared 27 women who remained premenopausal (plasma Follicle Stimulating Hormone ( FSH ) levels all less than 30 IU) and who were followed for at least one year, with 21 women who were classified as perimenopausal ( at least one FSH greater than 30 after a previous FSH less than 30) and who were followed for at least one year after their initial elevated FSH. The women who e remained premenopausal had no significant change in body composition as assessed by DEXA scanning while the perimenopausal women had significant increases in the amount of body fat as well as their percent fat. These women who are entering the earliest stage of the menopause also had significant lose of bone mineral from their lumbar spine while those women who remained premenopausal were actually significantly still increasing the amount of bone mineral in the spine. Both groups were losing bone mineral at the hip, with no acceleration yet seen in the perimenopausal group These changes all occured prior to the complete cessation of menstruation, and indicate that changes are occuring earlier in the transition across the menopause than previously thought.